Method and device for detecting period length fluctuations of periodic signals

ABSTRACT

To determine the period length of a first signal, the length is measured by counting the periods of a second signal with a shorter period length. To measure the fluctuations of the period length of the first signal whilst also taking into account the fluctuations of the period length of the second signal, the measurement is carried out for two different values of the period length of the second signal. Both the fluctuations of the period length of the first signal and the accumulated fluctuations of the period length of the second signal are calculated independently of one another from the two values. The method enables the period length fluctuations of a first signal that originates from a phase-locked loop to be detected.

PRIORITY CLAIM

This application is the national stage application of Internationalapplication number PCT/EP2003/008844, filed on Aug. 8, 2003, whichclaims the benefit of priority to German Patent Application 102 39099.1, filed on Aug. 26, 2002, incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a method as well as an apparatus fordetecting period length fluctuations of a periodic first signal and/orof a periodic second signal, whereby the period length of the secondsignal is shorter than that of the first signal and a reference numberof periods of the second signal, which arise during a certain number ofperiods of the first signal is obtained.

2. Background Information

In the case of periodic signals disadvantageously fluctuations of theperiod length also arise. This can lead for example in digitaltechnology to edge-controlled actions being triggered too early or toolate causing malfunctions to arise. If for example in the case of serialdata communication with data retrieval in a pre-determined cycle theanalog value or the digital state of a line must be scanned, periodlength fluctuations of the scanning rate can lead to errors arisingduring data retrieval, since scanning is carried out at an incorrecttime.

In order to measure the fluctuations of the period length of a periodicfirst signal, it is known to measure the duration of a period of thefirst signal several times by means of a periodic second signal, theperiod length of which is shorter than the period length of the firstsignal. In this case, a reference number of periods of the secondsignal, which arise during a period of the first signal and/or can beinscribed in a period is obtained. The period length of the first signalthereby corresponds to the reference number multiplied by the periodlength of the second signal. In this case, a quantization error arises,since the period length of the first signal is only resolved with anintegral number of periods of the second signal, the quantization errorfalling with decreasing period length of the second signal. In addition,period length fluctuations of the second signal also have an effect onthe measurement, whereby in relation to the second signal, theaccumulated period length fluctuations make it possible to realize thatin a measurement period the period length fluctuations of the referencenumber are added onto periods of the second signal. The period lengthfluctuations of the second signal therefore have a greater effect on themeasurement result, the higher the reference number.

FIG. 1 illustrates the basic principle of this measurement process. Inthis case, a reference number m is defined in periods of the secondsignal 2, which can be inscribed in a period of the first signal 1. Theperiod length of the first signal 1 is provided with the referencesymbol 3 and features period length fluctuations, which are shown asbroken lines. In known methods, in order to detect the period lengthfluctuations of the first signal 1, a signal 2 with very short periodlength fluctuations is used, so that the period length of individualperiods of the first signal 1 can be measured with high precision.Vice-versa if the signal 1 is present with high precision and very shortperiod length fluctuations, the accumulated or added period lengthfluctuations of the second signal can be detected. By multiplemeasurements of reference numbers m, the period length fluctuations ofthe first signal or the accumulated period length fluctuations of thesecond signal can therefore be measured, dependent on what signal withhigh precision and/or with short period fluctuations is present.

Disadvantageously, however, this requires at least a high precisionsignal with a very short period length fluctuation.

BRIEF SUMMARY

The underlying object of the present invention is to provide a method aswell as an apparatus with which period length fluctuations of a periodicfirst signal are measured at low cost by counting a reference number,inscribable in the period of the first signal, of periods of a higherfrequent periodic second signal without requiring any high precisionreference signals.

In accordance with the invention, this object is achieved by a methodwith the features of claim 1 and/or an apparatus with the features ofclaim 10. The sub-claims in each case define preferred and advantageousembodiments of the present invention.

In accordance with the present invention, the reference number ofperiods of the second signal, which can be inscribed in one or severalperiods of the first signal is obtained. In this case, the measurementof this reference number is carried out for two different period lengthsof the second signal. Dependent on the period length of the secondsignal also the reference number and the effect of the period lengthfluctuations of the second signal on the measurement precision change.

FIG. 2 shows for a first signal with a given period length the standarddeviation σ of several measurements of the period length of the firstsignal 1 by counting inscribable periods of the second signal 2 over thereference number m. The standard deviation σ therefore relates to theentire error of the measurement, which depends both on period lengthfluctuations of the first signal 1 and also on the accumulated periodlength fluctuation of the second signal 2. The reference number m isproportionally dependent on the reciprocal value of the period length 4of the second signal 2. The standard deviation σ is taken into accountas a measure for the period length fluctuations.

The curve in FIG. 2 can be divided into three zones A to C. In zone A onthe basis of the low reference numbers m the curve of the standarddeviation is essentially defined by the quantization error, which isproportional to the period length 4 of the second signal 2 and/orvice-versa proportional to the reference number m. In zone A the curveof the standard deviation falls with increasing reference numbers m andreaches its minimum in zone B.

In zone B the standard deviation is defined both by the period lengthfluctuations of the first signal 1 as well as of the second signal 2.With rising value of the period length fluctuations of the first signal1 the curve is raised and flattened out in the zone of the minimum.

Subsequently, the curve of the standard deviation rises with increasingreference numbers m. The curve in this zone essentially behavesproportionally to the square root of the reference number m.

In accordance with the invention, the measurement is now carried outseveral times for two different values of the period length 4 of thesecond signal 2. For a certain reference number m the standard deviationσ for the entire measurement is determined as followsσ² =a ²+2ayρ+y ²

For roughly normally split period length fluctuations σ₁ of the firstsignal 1 and σ₂ of the second signal 2 a regression coefficient r can becalculated as followsr=ρ·x,

whereby x=y/a, y=σ₁ and a=σ₂·√m

Determination of the standard deviation σ for the entire measurement atplaces m0 and m>m0 leads to the equations

$\sigma = {\sqrt{\sigma_{0}^{2} + {\sigma_{2}^{2}m_{0}\frac{m}{m_{0}}\left( {1 + {2r}} \right)\left( {1 - A} \right)}}\mspace{14mu}{and}}$$\sigma = \sqrt{{\sigma_{0}^{2}/A} - {{\sigma_{1}^{2}\left( {1 - A} \right)}/A}}$

for the period length fluctuations of the two signals 1, 2 and/or theirstandard deviations σ²·√{square root over (m₀)} equal to the accumulatedperiod length fluctuations of the second signal 2 and the period lengthfluctuations and/or the standard deviation of the first signal equal toσ₁ dependent on the measured quantities m₀, σ₀ and m, σ. The regressioncoefficient A is composed of the regression coefficients m₀, r₀ and m,r. In this case, A is calculated as follows

$A = {\left( \frac{m_{0}}{m} \right)\left( \frac{1 + {2r_{0}}}{1 + {2r}} \right)}$

In the following, the regression coefficients resulting thereby areshown for several ratios of m to m₀

m/m0 1.69 2.0 2.47 4.0 r 0.0323 0.0189 0.0094 0.0009 r ₀ = 0.1601 Ā0.7361 0.6387 0.5267 0.331

For greater clarity, FIG. 3 shows a diagram in which the standarddeviation σ_(B) of the entire measurement for a first period length ofthe second signal 2 is plotted towards the right and the standarddeviation σ_(C) of the entire measurement for a second shorter periodlength of the signal 2 is plotted upwards. A specific point in thisdiagram, for certain values of the standard deviation σ₁ of the periodlength fluctuation of the signal 1 and the accumulated standarddeviation σ₂ √{square root over (m)} of the period length fluctuationsof the second signal 2, reproduces the values for the standarddeviations σ_(B) and σ_(C) of the entire measurement error, which resultwhen the measurement is carried out with the two different periodlengths 4 of the second signal 2. As an example, two groups of lines aredrawn in the diagram, which represent the standard deviations of theentire measurement for the two period lengths of the second signal 2 inthe case of constant standard deviation σ₁ of the period fluctuation ofthe first signal 1 and/or in the case of constant standard deviationσ₂√{square root over (m)} of the accumulated period length fluctuationsof the second signal 2.

The substantially horizontal and straight running curve familyrepresents the case where the standard deviation of the accumulatedperiod length fluctuations of the second signal 2 is constant, thedirection in which the standard deviation of the accumulated periodlength fluctuations of the second signal 2 rise, being indicated withthe arrow pointing upwards, which is identified with σ₂ √{square rootover (m)}.

The other substantially perpendicularly running and upwards arched curvefamily represents the case where the standard deviation of the periodlength fluctuations of the first signal 1 is constant, the arrowpointing to the right, which is identified with σ₁, showing thedirection, in which the standard fluctuations of the period length ofthe first signal 1 rise. An essentially angle-bisecting auxiliarystraight line passing through the origin is also illustrated, the slopeof which is the reciprocal value of the root of the regressioncoefficient A and which represents the asymptote for the curve familyfor constant standard deviation σ₁ of the period length fluctuation ofthe first signal 1.

In accordance with the invention, the values for the standard deviationof the period length fluctuations of the first signal 1 as well as thestandard deviation of the accumulated period length fluctuations of thesecond signal 2 can now be read off in the diagram for 2 values σ_(B)and σ_(C) of the standard deviation of the entire measurement for twodifferent period lengths 4 of the second signal 2. For this purpose, thecurves σ₁=constant and σ₂√{square root over (m)}=constant, the point ofintersection of which marks the wanted place in the plane σ_(B), σ_(C)can be selected from the two curve families. These values can also bedetermined arithmetically using the equations

$\sigma_{acc} = {{\sigma_{2}\sqrt{m_{o}}} = {{\sqrt{\frac{\sigma^{2} - \sigma_{0}^{2}}{\frac{m}{m_{0}}\left( {1 + {2r}} \right)\left( {1 - A} \right)}}\mspace{14mu}\sigma_{1}} = \sqrt{\frac{\sigma^{2}A}{1 - A}}}}$

Through measurement with two different period lengths of the secondsignal 2 the period length fluctuations both of the first signal 1 andalso in accumulated form of the second signal 2 can therefore becalculated independently of one another. This means that advantageouslyno high precision reference signal with especially short period lengthfluctuations is required.

In this way, the period length fluctuation of the first signal and/orsecond signal can be detected at low cost, as a result of which themethod in accordance with the invention can also be implementedeconomically in integrated semiconductors as a self-test routine. Forexample, this is useful in the case of phase-locked loops, in order tobe able to check the period length fluctuations of the output signal.Such a phase-locked loop can be used in data transmission for dataretrieval in DSL data transmission systems for example.

To implement the method in accordance with the invention, it isbasically immaterial how often the measurement is carried out, sinceevaluation of the standard deviation as a measure for period lengthfluctuations concerns a statistical method, advantageously themeasurements for the two different period lengths of the second signal 2being repeated several times, in order to achieve more reliable results.Naturally, the process can therefore be run in an integrated circuit ora corresponding device controlled by a micro-program, for examplewhenever these are switched on.

Up to now, the case where the measurement is only carried out for twodifferent period lengths 4 of the second signal 2 has been described.However, in addition, it is also conceivable that three or moredifferent values for the period length 4 of the second signal 2 aremeasured, as a result of which other equations follow and under certaincircumstances a redundant equation system ensues, from which anoptimization function may be derived, in the case of which for examplethe values for the period length fluctuation of the first signal 1 andthe accumulated period length fluctuation of the second signal 2 can bedetermined according to the minimum error square method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the time-related curves of a first and a second signal withdifferent period lengths.

FIG. 2 shows the curve of the standard deviation of the measurement ofthe period length of a signal by counting periods of the second signalwith a shorter period length.

FIG. 3 shows, for various combinations of the period length fluctuationof the first and of the second signal, the values resulting frommeasuring different period lengths of the second signal, and

FIG. 4 shows the circuit diagram of an apparatus for calculating theperiod length fluctuations of both signals.

DETAILED DESCRIPTION

FIG. 1 in principle shows the curve of a periodic first signal 1 and aperiodic second signal 2, the first signal 1 having a period length 3,which is longer than the period length 4 of the second signal 2. Theperiod length 3 of the first signal 1 is to be determined by countingthe periods of the second signal 2, which arise during the period length3 of the first signal 1. The number of these periods is designated inthe following with m. In the case illustrated m=8.

The period length 3 of the first signal 1 in this case features certainfluctuations, as indicated by broken lines. Equally, the period length 4of the second signal 2 features certain fluctuations, which however arenot shown in FIG. 1.

In the following the case will be examined where the average periodlength of the first signal 1 is substantially constant and will bemeasured by means of a second signal 2 inscribable in the period length4. When the period length 4 of the second signal 2 falls and/or thefrequency of the second signal 2 rises, the number m, which isdesignated as reference number, must increase.

In FIG. 2 the curve of the error in this period length measurement isshown over the reference number, which vice versa is again proportionalto the period length 4 of the second signal 2. As a measure for theerror when determining the period length fluctuation according to theaforementioned method, the standard deviation a of the measurementresult, which ensues when the measurement is carried out several timesis plotted in FIG. 2 and below. FIG. 2 shows the curve split into threezones A, B and C. In zone A the period length 4 of the second signal 2is high, so that low values m result and therefore the standarddeviation σ is predominantly determined by the quantization error, whichvice versa behaves proportionally to the reference number m.Accordingly, the curve for σ falls in zone A with increasing m. In zoneB the curve of the standard deviation possesses a minimum, whereby inthis zone the standard deviation substantially equally depends on theperiod length fluctuations of the first signal 1 as well as the periodlength fluctuations of the second signal 2. In the case of the periodlength fluctuations of the second signal 2 it is to be stated that thisalways concerns accumulated period length fluctuations, since during ameasurement length several periods of the second signal 2 arise, theperiod length fluctuations of which are added together.

With increasing reference number m and/or falling period length 4 of thesecond signal 2 in zone C the curve of the standard deviation againrises, since in the case of high reference number m and/or high numberof counted periods of the second signal 2 the effect of the periodlength fluctuations of the second signal 2 becomes greater. In this zonethe curve behaves roughly proportionally to the square root of thereference number m.

To carry out the measurement two period lengths are now calculated,whereby the first period length is determined in such a way that thecorresponding reference number m during the measurement lies at theminimum of the curve of the standard deviation σ in zone B and thesecond value of the period length is lower and lies in zone C. For thetwo values of the period length 4 of the second signal 2 the periodlength measurement for the first signal 1 is now carried out severaltimes and two values for the standard deviation σ_(B) and σ_(C) areobtained, σ_(B) being the value of the standard deviation, which wasdetermined for the reference number m lying in zone B. The same appliesfor the value σ_(C).

FIG. 3 as already described above shows a diagram the surface plane ofwhich is covered by the values for σ_(B) and σ_(C). In this plane, theplaces, which in the plane of σ_(B) and σ_(C) ensue for certain valuecombinations for the standard deviation σ₁ of the period lengthfluctuation of the signal 1 and the standard deviation σ² √{square rootover (m)} of the accumulated period length fluctuations of the secondsignal 2, are indicated by way of two example curve families.

The substantially horizontal and straight running curve familyrepresents the curves on which the standard deviation σ² √{square rootover (m)} of the period length fluctuation of the second signal 2 isconstant, whereas the curve family running substantially perpendicularand arched upwards indicates the places at which the standard deviationsigma.1 of the period length fluctuation of the first signal 1 isconstant. The arrow pointing to the right and designated with sigma. 1indicates the direction in which the curves with a higher standarddeviation of the period length fluctuation of the first signal 1 lie.The same applies for the arrow pointing upwards and designated with σ²√{square root over (m)} for increasing values of the standard deviationof the accumulated period length fluctuation of the second signal 2.

For the graphic solution the place, which results from the two measuredvalues for σ_(B) and σ_(C) for the two different period lengths of thesecond signal 2, can be marked in this diagram shown in FIG. 3.Subsequently, it is determined which two curves of both curve familiesintersect at this place and depending thereon the value sigma.1 for thestandard deviation of the period length fluctuations of the first signal1 and the value σ²√{square root over (m)} for the accumulated periodlength fluctuations of the second signal 2 can be established.

FIG. 4 shows an arithmetical solution, whereby the case is illustratedwhere the first signal 1 is the output signal of a phase-locked loop 5and the second signal 2 is the output signal of a reference oscillator6, the period length and/or frequency of which can be varied and/or atleast switched between two values. The two signals 1, 2 are fed to acounter 7, which can count the number of periods of the second signal 2,that can be inscribed in a period of the first signal 1 and/or ariseduring this time-span. The circuit arrangement shown in FIG. 4 alsocomprises a (not illustrated) control device, which controls the variouscomponents in a suitable way. The measurements are carried out by thecounter 7 for the two different values for the period length of thesecond signal 2 and passed onto a tabulating block 8 in which thestatistical evaluation takes place. To this end for the two differentperiod lengths of the second signal 2 the standard deviation of themeasurement results and/or the counter figures calculated by the counter7 are formed respectively in the tabulating block 8. The tabulatingblock 8 therefore calculates the two values σ_(B) and σ_(C), which itpasses onto an evaluation circuit 9. From the two values σ_(B) and σ_(C)this calculates the squared standard deviation and/or variance σ₁ ² andσ₂ ² for the first signal 1 and/or the second signal 2. In this case,the reference number m is considered in the evaluation block 9 andregression coefficients CB and CC which have been previously calculatedarithmetically or experimentally and stored in the apparatus are alsotaken into account.

The apparatus shown in FIG. 4 can be implemented in an integratedcircuit for example and an estimate of the period length fluctuations ofthe first signal 1 of the phase-locked loop 5 can be carried outwhenever the integrated circuit is switched on as self-test routine.

It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, that areintended to define the spirit and scope of this invention.

1. A method for detecting period length fluctuations of at least one ofa periodic first signal and of a periodic second signal in which aperiod length of the second signal is shorter than a period length ofthe first signal and a reference number of the periods of the secondsignal that arise in the time-span of a base number of periods of thefirst signal is obtained, the method comprising: determining a firstreference number for a first period length of the second signal and asecond reference number for a second period length, different from thefirst period length, of the second signal; and calculating a measure forthe period length fluctuation of at least one of the first signal and ofthe second signal dependent on the first and second reference number. 2.The method according to claim 1, further comprising determining at leastone reference number of the first and second reference numbers severaltimes.
 3. The method according to claim 2, further comprisingdetermining a standard deviation of period lengths of a signal of thefirst and second signals as a measure for the period length fluctuationof the signal.
 4. The method according to claim 1, further comprisingselecting the first period length of the second signal such that theeffect of the period length fluctuation of the first signal is greaterthan the effect of the period length fluctuation of the second signal onthe first reference number.
 5. The method according to claim 1, furthercomprising selecting the second period length of the second signal sucha manner that the effect of the period length fluctuation of the firstsignal is less than the effect of the period length fluctuation of thesecond signal on the second reference number.
 6. The method according toclaim 1, wherein the base number of periods of the first signal is
 1. 7.The method according to claim 1, wherein the first signal is an outputsignal of a phase-locked loop and the second signal is an output signalof a reference oscillator.
 8. The method according to claim 1, furthercomprising carrying out the method in an integrated semiconductor as aself-test.
 9. The method according to claim 1, wherein at the start ofdetermining a reference number, the first signal and the second signalare in phase.
 10. The method according to claim 1, further comprisingdetermining at least one of the period length fluctuation of the firstsignal and of the second signal dependent on previously calculatedregression coefficients.
 11. An apparatus for detecting period lengthfluctuations of at least one of a periodic first signal and a periodicsecond signal in which a period length of the second signal is shorterthan a period length of the first signal, wherein the apparatuscomprises an evaluation circuit that: determines a reference number fromperiods of the second signal that arise in the time-span of a basenumber of periods of the first signal; and determines a first referencenumber for a first period length of the second signal and a secondreference number for a second period length, different from the firstperiod length of the second signal, dependent on the first referencenumber and the second reference number, and wherein a measure of theperiod length fluctuation of at least one of the first signal and of thesecond signal is determined.
 12. The apparatus according to claim 11,further comprising a reference oscillator for producing the secondsignal.
 13. The apparatus according to claim 11, further comprising aphase-locked loop, wherein the apparatus designed such that an outputsignal of the phase-locked loop is the first signal.
 14. The apparatusaccording to claim 11, wherein the apparatus is an integratedsemiconductor.
 15. An apparatus according to claim 11, wherein at leastone reference number of the first and second reference numbers isdetermined several times.
 16. An apparatus according to claim 15,wherein as a measure for the period length fluctuation of a signal ofthe first and second signals the standard deviation of period lengths ofthe signal is determined.
 17. An apparatus according to claim 11,wherein the first period length of the second signal is selected suchthat the effect of the period length fluctuation of the first signal isgreater than the effect of the period length fluctuation of the secondsignal on the first reference number.
 18. An apparatus according toclaim 11, wherein the second period length of the second signal isselected such that the effect of the period length fluctuation of thefirst signal is less than the effect of the period length fluctuation ofthe second signal on the second reference number.
 19. An apparatusaccording to claim 11, wherein the base number of periods of the firstsignal is 1.